Limited heat in one zone, circulator running, air lock?

We woke up this morning and in our master bedroom, the thermostat read 63 even though it's set at 67. It was single digits cold here overnite but that has happened before and not caused an issue. Our primary winter heat system is an oil fired hot water boiler. The thermostat for the AC/Heat pump (currently that system is off, just still reads temp) also said 63 so it's not a thermostat issue I don't think.

Zone relay is on and circulator feels like it's running (hand on it, get the slight vibration).

I believe our circulators act in a "pull" fashion vs a "push" fashion but I'm not 100% sure (I can't post a picture if need be).

Anyway, the pipe coming out the top of the boiler goes to the valves for the zones in the house. Right up near there, the pipe is really hot as expected (water coming right off the boiler). On the circulator side, it's warm but you can hold your hand on it. I can also keep my hand on the baseboards where I normally can't. It's like some hot is getting through but not well, making us suspect an air lock somewhere in that zone.

Right above the circulator, we have a valve that can be opened to let water out of the zone so we did drain some in to a bucket but didn't hear or see any air coming out. We had that happen in another zone years ago and when you opened the valve, you knew there was air in it because it spit and sputtered. We weren't sure how much water we could take out though and only drained about 2" into a small bucket. This is a long zone because it goes from the basement up to the second floor and the circulates around the master bedroom and bathroom (4 radiators in total). Water automatically gets added to the zone right? So should we maybe drain the whole thing or at least a good bit more and see what happens?

My husband thought that the piping right above the circulator got a bit warmer after we drained some water.

To add to this, we closed all other zone valves off and drained "everything" out of the offending zone. It actually sounded like when the water stopped, we had a pocket of air up in there somewhere but left it open for a while and nothing else came out so we shut the hose fitting valve and let the circulator run (first with other zones closed and then with them opening and calling for heat).

Before we did the final drain I took temp readings on the fins of the baseboards. There are 4 involved and frankly, we've always thought that the area was too big and needed more especially in the front of the bedroom with the dormer window where the floor is always cold. Anyway, in the mater bath, which is the first radiator off the boiler, I stopped reading at 140 and still going up, the one in the master closet, just about the same. Out in the bedroom, big radiator on the outside side wall, it was about 10deg cooler after the same period of holding the thermometer there. Then I tried the last radiator in the leg, in the front of the dormer where we always think it's cold, about 115deg there... still creeping up but not much. The boiler is set to max = 180, min = 160 so I'd expect to be in the 150deg range at the farthest point out figuring for heat loss through the loop and I actually took a reading in another zone and confirmed that (probably anecdotal tho). For right now, we put everything back to normal to see what it does. We have had really cold temps before and don't recall having this happen. I'm wondering tho if going to a max of 200deg on the boiler might help? It'll run longer but then temp won't drop as much over the run and perhaps help?

Yep I'm sure of that. And I'm getting gurgling in other zones a bit now tho things are at the temp they're supposed to be. This master bedroom zone still hasn't caught up but seems to be getting better.

I really think it just can't handle the big open room over the garage (which we keep at 55 on its own zone). The bathroom baseboard and the one in the closet between it and the bedroom seem to be ok but then the temp drops off on the two in the bedroom. We have a storage area behind the bedroom wall about 6' wide before the outside wall and we think the pipe from the closet radiator goes into that cold space to get to the outside side wall to get to the #3 radiator. That's darn cold in that area and I'm not sure how well its insulated (if at all) under the plywood that's down as a floor (this is unfinished space). We're contemplating unscrewing the plywood and wrapping the pipe if we can find it. It's a pretty long run so I'd imagine it could cause some heat loss.

Also - I'm wondering if baseboards could be added in the bedroom to help with the cold. Our main zone downstairs is all one zone with 9 radiators of varying length on it so it would seem that having a zone with 4 upstairs could handle more, but of course, that upstairs zone might have more heat loss so maybe you can't add to it. It just struck us as odd as far as how many radiators were on each zone.

Are all of the baseboards in series, or in parallel? If series, it would get colder through each one and since most baseboards are not as effective in generating convection currents when they are cool, it would be expected for the last one to be colder and not heating well. If in parallel, at least the inlet should be very close to the same temperature everywhere, especially if you have any insulation around the supply pipes. But, even if not, unless it is running through unheated space, it shouldn't lose huge amounts of heat while getting to the baseboard.

If you don't have an autofill valve, draining some water can deplete the system of needed water. Normally, it's not a great idea to drain a system unless you need to do maintenance on it as fresh water adds chlorine, minerals, and oxygen into the system that it doesn't need. Depending on how the pipes are arranged, it may take a lot of flow to purge all of the air out of the system.

A system generally can only 'gurgle' if it isn't full, IOW there's some air in the system. High flow rates can make some noise, but it usually isn't gurgling. ANd, you shouldn't have high enough flow rates to cause turbulance, otherwise you'd wear things out.

My belief is that they're in series just based on the amount of piping I can see for the first floor zone while I'm in the basement looking up but I will check that again later today. I may also have some pictures of that phase while the house was being built. That may also explain why the front 3 rooms on the main floor always seem a few degrees colder as well, there are 9 baseboards on that zone.

The master bedroom zone does run through some unheated space in that storage area behind the bedroom next to the closet (basically just a space made with a knee wall so that the ceiling in the bedroom isn't 4' tall LOL) We don't know if they put insulation under the plywood pseudo floor that's down there. Tho, I would think because it's above the ceiling of our powder room and laundry room they would have but nothing with our idiot builder would surprise me.

We are on well water and we do have an autofill valve. At least I believe that's what the big fitting above the extrol tank with the air purge valve above it. I also put a tire pressure guage on the tank and it's reading right around 13 - 14psi which if I recall, is normal. I checked the air purge valve and it's not tight on the screw threads which I also believe is how it's supposed to be.

The other 2nd story zone was gurgling a bunch last nite and unfortunately, there's no way to shut off the supply side there after the check valve (did I mention idiot builder, since all other check valves he put in have had to be replaced, we've added shut offs as we've gone but this one section has been ok), only the return size but we also got some bubbles out of that by opening the valve a bit.

This morning in the master bedroom, in the dormer window part, it's about 1.5 degrees cooler than the rest of the room, but the thermostat was satisfied at 67 and the AC/heat pump thermo read 67, not 63 like yesterday morning so I think our futzing around yesterday at least did something since the outside temp this morning is about the same as yesterday moring (8degF right now). We do have less wind today tho and we've noticed wind always seems to make the alcove cooler and we're hoping to figure out a way to better insulate that without ripping off siding and sheathing.

The ripping it out seems like a better option for two reasons. One, looking better like you said and 2, don't need to cut up the carpet and subfloor as I assume it could be fitted to the existing pipe that comes up through the floor already?